FGF21 and mitochondrial disease: In the past 5 years, serum fibroblast growth factor-21 (FGF-21) and serum growth and differentiation factor-15 (GDF-15) (Kajiyama et al., 1989; Suomalainen et al., 2011; Davis et al., 2013; Kalko et al., 2014; Fujita et al., 2015; Yatsuga et al., 2015) have emerged as two promising diagnostic biomarkers for mitochondrial diseases.